I bought some decades ago to do the same thing, but never got around to it. I was going to try to mod them for 6.5" speakers and needed a plan for how to mod the buckets, but in the modern day, it would make more sense to 3D print some custom ones rather than mod the OEM ones as I wanted to use the OEM grilles which are flat against the trim panel.
I can't find them at the moment or I'd snap a pic. As for prewired, my 89 wasn't prewired, but different years might do things differently.